Encrypted voice and data systems are well known. Many of these systems provide secure communications between two or more users by sharing one or more pieces of information between the users, thereby permitting only those users knowing the information to properly decrypt the message. Generally speaking, an encryption algorithm is used by peer devices to encrypt or decrypt voice and data messages. The encryption algorithm is a nonlinear mathematical function defined by an initial starting vector and a key variable (or “key”) that generates a pseudo-random sequence, known as a keystream. The keystream is XORed (exclusive “or” function, as known in the art) with plain (unencrypted) text to generate cipher text. The cipher text is transmitted to a receiver over a communication channel, which may comprise, for example, a radio frequency (RF) channel. The receiver XORs the received cipher text with a keystream, generated from the same key and encryption algorithm as used by the transmitter, yielding a plain text (decrypted) output.
It is well known that proper decryption can not occur unless a receiver achieves synchronization with the transmitter, i.e., to lineup its encryption stage with the encryption stage of the transmitter. To that end, encryption synchronization (also known as e-sync) information is periodically sent over the communication channel to initiate and maintain synchronization between a transmitter and one or more receivers. As will be appreciated, there are a variety of possible modes of operation for sending such e-sync information, depending on characteristics of the RF channel, modulation type(s), bandwidth limitations and the like.
Generally, in TDM systems, e-sync bit(s) are interleaved among data blocks intended for particular receiver(s), wherein the data blocks form part of a TDM slot. Typically, both the data blocks and slots include a fixed number of bits encoding a plurality of modulation symbols (e.g., 16-QAM symbols). The number of bits per symbol (and hence the number of symbols per block) varies according to the type of modulation employed. Most advantageously, the e-sync bit(s) are sent once every several slots (as opposed to each slot), yet within a predetermined maximum delay period for late entry, to achieve an optimal tradeoff of bandwidth utilization versus quality. A receiver obtains initial synchronization by looking at consecutive slots until it finds an e-sync block that identifies a starting vector for its encryption algorithm. Once the encryption state is initialized, a receiver maintained e-sync by advancing its encryption state corresponding to the number of received bits. Historically, the type of modulation did not vary from block to block, thus a receiver could maintain e-sync by simply counting the number of received modulation symbols, or the amount of time elapsed, and advancing a linear feedback shift register (LFSR) sequence a fixed number of bits for each received symbol.
Recent advances in technology have produced TDM systems that are eligible to use different modulation types within different blocks of the same or different slots, and wherein different blocks may be destined for different receivers. One such system is described in U.S. patent application Ser. No. 09/760,981, titled “Slot Format and Acknowledgment Method for a Wireless Communication System,” filed Jan. 16, 2001, incorporated herein by reference in its entirety. Generally, in such system, the number of bits per block is fixed, but the number of bits per slot will vary according to the modulation type(s) used within the slot.
A problem that arises is that channel impairments introduced during transmission may cause the receiver to be unable to successfully decode all or portions of one or more blocks. Consequently, in a multi-modulation TDM system, a receiver may not know what type of modulation symbols are used within certain blocks (and hence the number of bits per slot) or whether it is the intended receiver for certain blocks. As a result, the receiver does not know how many bits to advance its encryption state from block to block to maintain encryption sync.
Accordingly, there is a need for a method and apparatus that facilitates maintaining e-sync between a transmitter and one or more receivers in a multi-modulation TDM system. Advantageously, the method and apparatus will provide for the receivers advancing their encryption state to maintain e-sync without necessarily having knowledge of the number(s) and type(s) of modulation symbols within each block or the number of bits used from slot to slot, and without relying on peer-to-peer messaging. The present invention is directed to addressing these needs.